The present invention relates to a system for compensating for errors in registration between colours printed in a multi-colour printing machine.
In a conventional printing machine, colour registration can be ensured in two different ways, depending upon the design of the machine:
(1) The machine is equipped between two successive printing units with a registering or web length compensating device including a roll around which the web is looped and whose position can be adjusted in order to increase or reduce the length of web extending between the two successive printing units being considered;
(2) the machine is equipped with a differential device associated with the driving mechanism of the printing cylinder of one printing unit, so that compensation is achieved by varying the angular position of the printing cylinder of one printing unit in relation to the previous or succeeding one.
In both cases, when compensation is effected, the tension in the web is altered positively or negatively, according to the sense of compensation. Under normal operating conditions, the web is subjected to an equilibrium tension To, which is dependent upon the dimensions of the printing cylinder, on the tautness of the material forming the web and on the tension of the web at the inlet of the machine. After the tension of the web has been modified during the compensating action, it returns to the equilibrium tension To, in accordance with an exponential law, where the printed material is such that deformations are proportional to constraints.
If one considers a conventional machine in which colour registration is ensured using a registration or web length compensating device and if the devices for reading reference marks carried by the web and associated with various colours detect an error in registration between two colours applied by two successive printing units which is equal to one unit of length, a control unit transmits an order to move the roll of the registering or length compensating device by a distance equal to 1/2 the unit of length, in order to increase or reduce the length of the web between the two printing units by one unit of length. The resulting unit variation in length produces a variation x in the tension of the cloth such that x=K1/L, K being the tautness of the web material and L the length of web between the two printing cylinders. The instantaneous value which the tension of the web assumes is therefore equal to To.+-.x, which must always be between an upper tension limit TM and a lower tension limit Tm. Theoretically, the lower tension limit is O, but in practice one cannot fall below a finite lower tension limit, for reasons of behaviour of the web, particularly as a result of the passive resistance of the return cylinders of the machine. TM, defining the upper tension limit corresponds substantially to the elastic limit of the web, because if the elastic limit is exceeded the web undergoes permanent elongation, which brings with it two principal disadvantages, that is to say on the one hand, the finished printed product does not have the correct length, and on the other hand, the elongated portion which appears during the running of the machine is liable not to have sufficient tautness to ensure tension of the web and guidance of it in the machine. Certain traditional web materials (paper, cardboard, plastic or thick aluminium film, for example), possess a sufficiently high elastic limit for TM to be very much above zero, and therefore registration compensations of normal amplitude are possible.
On the other hand, materials which herein will be referred to as light materials, such as a thin aluminium or polyurethane films, for example, have an elastic limit not greatly above zero tension. There thus remains a small margin in which to carry out registration compensation, and the permissible amplitude becomes incompatible with industrial use of the machine.